1. Field of the Invention
This invention relates a method for the detention and correction of image distortions in medical imaging during which a plurality of images are formed. The invention also relates to a computer tomography apparatus (referred to hereinafter as CT apparatus) which includes and X-ray detector arrangement, and an arithmetic unit for processing measured signals and for calculating slice images from the measured signals in order to carry out such a method.
2. Description of the Related Art
A method for the correction of image distortions in computer tomography is known, for example from EP-A2-479 618. Therein, so-called geometrical distortions (distortions due to the special construction geometry of the image intensifier) and magnetic distortions (distortions caused by the terrestrial magnetic field) are corrected by means of a calibration object. The calibration object is positioned on the surface of the image intensifier and irradiated by X-rays so that a slice image is formed of the calibration object. The calibration object consists of a grid of copper wires. A horizontal and a vertical correction table are formed from the slice image of the calibration object; these tables describe said distortions and are used for the correction of CT images at a later stage. CT images may be taken, for example perpendicularly or obliquely to the slice images and are acquired from measuring values in different slice images.
The known method, however, detects exclusively image distortions which occur before irradiation of a patient. Exclusively these image distortions can be corrected in the images at a later stage. Distortions occurring directly during the exposure are not taken into account. However, the occurrence of distortions in the images cannot be precluded, despite the careful mechanical calibration of the computer tomography apparatus or other imaging system. Particularly the use of computer tomography during interventional surgery, for example neurosurgery, imposes extreme requirements as regards the precision, for example, to enable determination of the exact location of a tumor in the brain. Therefore, distortions should absolutely be detected and corrected.
Hereinafter some types of distortions and their possible causes will be described. Because of inaccurate calibration, the adjustment of the angle between the irradiation plane and the plane of the patient table during the exposure may be incorrect. During CT usually slices extending perpendicularly to the longitudinal axis of the body of a patient are recorded; the irradiation plane should then extend perpendicularly to the plane of the patient table. An adjustment inaccuracy amounting to no less than+ or -2.degree. for a desired angle of 90.degree. is then typical. Such incorrect adjustment causes shear-like distortion of the image.
A further type of distortion is the scale distortion which may occur in the image due to lack of correspondence between the actual and the intended speed of displacement of the patient table. Distortions thus arising cause stretched or compressed images of the area to be imaged.
The weight of the patient may cause bending of the patient table, thus giving rise to an incorrect angle between the irradiation plane and the patient table as well as to a further distortion effect which can generally be described by a quadratic equation.
Moreover, any rotation and translatory shift occurring between the coordinate system in which the (real) phantom body is situated and the image coordinate system in which the image of the phantom body (the phantom image) is situated are to be taken into account and corrected.